IAJPS 2016, 3 (7), 738-749
Narender Boggula et al
CODEN (USA): IAJPBB
ISSN 2349-7750
ISSN: 2349-7750
INDO AMERICAN JOURNAL OF
PHARMACEUTICAL SCIENCES Available online at: http://www.iajps.com
Research Article
SYNTHESIS, CHARACTERIZATION AND ANTI BACTERIAL ACTIVITY OF NOVEL 2-MERCAPTOBENZOXAZOLE DERIVATIVES Narender Boggula*, Ridhika M Patel, Akhil Rapolu, Pramod Kumar Bandari, Krishna Mohan Chinnala Nalla Narasimha Reddy Education Society’s Group of Institutions, School of Pharmacy Hyderabad, Telangana, India Abstract Benzoxazole and its derivatives are important class of bioactive molecules, their importance is due to their versatile application in the field of drugs and pharmaceuticals as well as in chemical systems. It finds use in research as a starting material for the synthesis of larger, usually bioactive structures. A new series of Benzoxazol-2-ylthio-N’-(4substituted) acetohydrazide derivatives (IV a-k), were obtained by synthesising new schiff’s bases derived from benzoxolyl-2-mercaptoacetohydrazide derivatives by treating with various aryl/hetero aryl aldehydes. Their chemical structures have been confirmed by 1H-NMR, 13C-NMR, FT-IR and Mass spectral data. The synthesised derivatives (IV a-k) were screened for their in-vitro anti bacterial activities by agar diffusion method. Among the synthesized derivatives IVb, IVe, IVk, IVi showed significant antibacterial activity. Key Words: Benzoxazole, heterocyclic compounds, aryl/hetero aryl aldehydes, schiff’s bases, anti bacterial activity.
Corresponding Author: Narender Boggula, Asst. Professor, School of Pharmacy, Nalla Narasimha Reddy Education Society’s Group of Institutions, Chowdariguda, Korremula, Ghatkesar, Ranga Reddy, Telangana, INDIA-500088. E-Mail: narender.b987@gmail.com Mobile: +91 966 655 2211
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Please cite this article in press as Narender Boggula et al, Synthesis, Characterization and Anti Bacterial Activity of Novel 2-Mercaptobenzoxazole Derivatives, Indo Am. J. P. Sci, 2016; 3(7).
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IAJPS 2016, 3 (7), 738-749 INTRODUCTION: Heterocyclic compounds are those which have a cyclic structure with two or more different kinds of atoms in the ring. The study of heterocycles is an evergreen field in organic chemistry. Heterocyclic chemistry has been progressing owing to their natural occurrence, specific chemical reactivity and broad spectrum utility. The heterocyclic compounds are very widely distributed in nature and are essential to living organisms. They play a vital role in the metabolism of all the living cells. Among large number of heterocycles found in nature, nitrogen heterocycles are the most abundant specially those containing oxygen or sulphur due to their wide distribution in nucleic acid illustration and their involvement in almost every physiological process of plants and animals. One of the main objectives of organic and medicinal chemistry is the design, synthesis and production of molecules having value as human therapeutic agents. During the past decade, combinatorial chemistry has provided access to chemical libraries based on privileged structures, with heterocyclic structures
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receiving special attention as they belong to a class of compounds with proven utility in medicinal chemistry. Biologically active benzoxazole derivatives have been known for long time, since they are the isosteres of naturally occurring cyclic nucleotides and they may easily interact with the biopolymers of the organisms. Benzoxazoles and its derivatives are important class of bioactive molecules, their importance is due to their versatile application in the field of drugs and pharmaceuticals as well as in chemical systems. Benzoxazole finds use in research as a starting material for the synthesis of larger, usually bioactive structures5. The benzoxazoles are a large chemical family used as antimicrobial agents against a wide spectrum of microorganisms. The high therapeutic activity of the related drugs has encouraged the medicinal chemists to synthesize a large number of novel chemotherapeutic agents. The incorporation of the benzoxazole nucleus is an important synthetic strategy in drug discovery. This class of molecules have broadened the scope in remedying various dispositions in clinical medicine.
MATERIALS AND METHODS: Scheme:
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Step 1: Preparation of benzo[d]oxazole-2-thiol (I) 10.91g of 2-aminophenol, 6.19ml of carbon di-sulphide, 5.65g of potassium hydroxide, and 15 ml of water were taken in a 250ml RBF and refluxed in 100 ml of 95% ethanol for 3 to 4 hours. Later charcoal was added cautiously, refluxed for 10 minutes and filtered. The filtrate was heated up to 70-800C and 100ml warm water was added, 5% glacial acetic acid was added and stirred vigorously. The product was obtained as crystals and placed in refrigerator for 3 hrs, for further crystallization. The product was filtered and dried. The dried product was recrystallized with ethanol. Step 2: Preparation of ethyl-2-(benzo[d]oxazol-2-ylthio) acetate (II) Equimolar quantities of benzo[d]oxazole-2-thiol (I) and ethylchloroacetate were refluxed in ethanol for 6-8 hrs. The reaction mixture was poured in to ice cold and stirred vigorously. The precipitate obtained was filtered, washed with water and dried. The product was recrystallised with ethanol. Step 3: Preparation of 2-(benzo[d]oxazol-2-ylthio) acetatohydrazide (III) Equimolar quantities of ethyl-2-(benzo[d]oxazol-2-ylthio) acetate (II) and 99% hydrazine hydrate were dissolved in methanol and kept aside for 20-22 hrs. The precipitate obtained was filtered, washed with cold alcohol and dried. Step 4: General preparation of Schiff’s bases (IV a-k) An appropriate aromatic aldehyde and 2-(benzo[d]oxazol-2-ylthio) acetatohydrazide (III) were dissolved in methanol, 1-2 drops glacial acetic acid was added and refluxed for 5-6 hrs. The mixture was kept in refrigerator overnight. The product obtained was filtered, dried and purified by recrystallization from ethanol.
RESULTS AND DISCUSSION: Table 1: Physical data of the 2-mercaptobenzoxazole Schiff’s bases (IV a-k) CODE Ar Mol. Formula Mol. Weight
%Yield
M.P (0C)
IVa
C17H15N3O2S
325
86
158-160
IVb
C16H12N3O2SCl
345.80
88
220-224
IVc
C17H15N3O3S
341.38
72
169-171
IVd
C18H17N3O4S
371.41
58
198-200
Continue………………
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IVe
C14H11N3O3S
301.32
89
186-189
IVf
C16H13N3O3S
327.36
82
208-210
IVg
C18H17N3O2S
339.41
66
176-178
IVh
C17H15N3O4S
357.38
78
205-208
IVi
C16H12N4O4S
356
69
217-220
IVj
C18H17N3O4S
371.09
78
160-162
IVk
C18H16N4O2S
352.4
76
210-212
Characterization Data: 2-(benzo[d]oxazol-2-ylthio)-N′-(4-methylbenzylidene)acetohydrazide (IVa):
Mol. Formula
: C17H15N3O2S
Mol. Weight
: 325
Solubility
: Freely soluble in Methanol & Alcohol
TLC solvent
:
n-Hexane: Ethyl acetate (3:7)
IR, Cm-1 (KBr) : 3424.51 (N-H, str.), 3029.80 (Ar-H, str.), 1736.85 (C=O, str. amide), 1621.71 (C=N), 1567.68 (N-H, out of plane), 1174.81 (C-S). H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 7.5 to 7.1 (m, 8H, Ar-H), 3.85 (s, 2H, S-CH2), 2.35 (s, 3H, methyl). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N), 161.04-114.18 I
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(Aromatic carbons), 143.208(C=N), 55.364, 40.9 (S-CH2). 24.3 (Methyl) Mass : m/z = 324 (M-1) 2-(benzo[d]oxazol-2-ylthio)-N′-(4-chlorobenzylidene) acetohydrazide (IVb):
Mol. Formula
: C16H12N3O2SCl
Mol. Weight
: 345.80
Solubility
:
Freely soluble in Methanol & Alcohol
TLC solvent
:
n-Hexane: Ethyl acetate (3:7)
IR, Cm-1 (KBr) : 3424.92 (N-H, str.), 3048.19 (Ar-H, str.), 1872.17 (C=O, str. amide), 1624.32 (C=N), 1592.04 (N-H, out of plane), 1168.01 (C-S). H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.01(s, 1H, NH-N), 7.6 to 7.26 (m, 8H, Ar-H), 3.82 (s, 2H, S-CH2). 13 C-NMR, δ ppm : 173(C=O), 165 ( Benzoxazole C=N),150.04-110.18 (Aromatic carbons), 143.208(C=N), 40.09 (S-CH2). Mass : m/z = 346 (M+1) 2-(benzo[d]oxazol-2-ylthio)-N′-(4-methoxybenzylidene) acetohydrazide (IVc): I
Mol. Formula : C17H15N3O3S Mol. Weight : 341.38 Solubility : Freely soluble in Methanol & Alcohol TLC solvent : n-Hexane: Ethyl acetate (3:7) IR, Cm-1 (KBr) : 3454.85 (N-H, str.), 3037.34 (Ar-H, str.), 1731.76 (C=O, str. amide), 1618.95(C=N), 1601.59(N-H, out of plane), 1250.61 (CO, str. ether), 1165.76 (C-S). I H-NMR, δ ppm : 8.61 (s, 1H, N=CH), 7.798(s, 1H, NH-N), 7.78 to 6.94(m, 8H, ArH), 3.85 (s, 2H, S-CH2), 3.77 (s, 3H, methoxy). 13 C-NMR, δ ppm : 174.187(C=O), 161.970 (Benzoxazole, C=N),161.04-114.18 (Aromatic carbons), 143.208(C=N), 55.364 (Methoxy), 41.061(S- CH2). Mass : m/z = 340 (M-1) 2-(benzo[d]oxazol-2-ylthio)-N′-(3,4-dimethoxybenzylidene)acetohydrazide(IVd):
Mol. Formula
:
Mol. Weight
: 371.41
Solubility
:
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C18H17N3O4S
Freely soluble in Methanol & Alcohol
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TLC solvent : n-Hexane: Ethyl acetate (3:7) IR, Cm-1 (KBr) : 3423.76 (N-H, str.), 3073.54 (Ar-H, str.), 1729.98 (C=O, str. amide), 1623.18 (C=N), 1598.50 (N-H, out of plane), 1258.73 (C-O-C, ether), 1140.11 (C-S). I H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 7.5 to 6.7(m, 7H, Ar-H), 3.85 (s, 2H, S-CH2), 3.73 (s, 6H, methoxy). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N), 162.31-114.18 (7C, Ar-C), 143.208(C=N), 56.21 (Methoxy), 40.9 (S-CH2). Mass : m/z = 371 2-(benzo[d]oxazol-2-ylthio)-N′-(furan-2-methylene)acetohydrazide (IVe):
Mol. Formula
:
C14H11N3O3S
Mol. Weight
:
301.32
Solubility
:
Freely soluble in Methanol & Alcohol
TLC solvent
:
n-Hexane: Ethyl acetate (3:7)
IR, Cm-1 (KBr) : 3434.6 (N-H, str.), 3109.65 (Ar-H, str.), 1770.33 (C=O, str. amide), 1644.02(C=N), 1147.44 (C-S). I H-NMR, δ ppm : 7.6 (s, 1H, N=CH), 7.37 (s, 1H, NH-N), 7.30 to 6.43(m, 7H, Ar-H), 3.83 (s, 2H, S-CH2). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N), 161.04-109.5 (Aromatic carbons), 134.7(C=N), 40.9 (S-CH2). Mass : m/z = 300.9 (M-1) 2-(benzo[d]oxazol-2-ylthio)-N′-(4-hydroxybenzylidene)acetohydrazide (IVf):
Mol. Formula
:
C16H13N3O3S
Mol. Weight
:
327.36
Solubility
:
Freely soluble in Methanol & Alcohol
TLC solvent
:
n-Hexane: Ethyl acetate (3:7)
IR, Cm-1 (KBr) : 3043.65 (Ar-H, str.), 1715.38 (C=O, str. amide), 1689.99(C=N), 1623.78 (N-H, out of plane), 1147.59 (C-S). I H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 7.5 to 6.8 (m, 8H, Ar-H), 5.0 (s, OH), 3.85 (s, 2H, S-CH2). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N),160.08-116.0 (Ar-C), 143.208(C=N), 40.9 (S-CH2). Mass : m/z = 326.21(M-1)
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2-(benzo[d]oxazol-2-ylthio)-N′-(4-(dimethylamino)benzylidene)acetohydrazide (IVg):
Mol. Formula
:
C18H17N3O2S
Mol. Weight
:
339.41
Solubility
:
Freely soluble in Methanol & Alcohol
TLC solvent
:
n-Hexane: Ethyl acetate (3:7)
IR, Cm-1 (KBr) : 3082.40 (Ar-H, str.), 1891.11 (C=O, str. amide), 1602.15 (N-H, out of plane), 1165.76 (C-S). I H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 7.5 to 6.6(m, 8H, Ar-H), 3.85 (s, 2H, S-CH2), 2.85(s, 6H, N,N-dimethyl). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N), 158.33-114.18 (9C,Ar-C), 143.208(C=N), 38.47(N,N-dimethyl), 40.9 (S-CH2) Mass : m/z = 339.41 (M+1) 2-(benzo[d]oxazol-2-ylthio)-N′-(3-hydroxy-4-methoxybenzylidene) acetohydrazide (IVh):
Mol. Formula : C17H15N3O4S Mol. Weight : 357.38 Solubility : Freely soluble in Methanol & Alcohol TLC solvent : n-Hexane: Ethyl acetate (3:7) IR, Cm-1 (KBr) : 3434.85 (N-H, str.), 3037.34 (Ar-CH, str.), 1731.76 (C=O, str. amide), 1618.95(C=N), 1165.76 (C-S). I H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 7.5 to 6.6 (m, 7H, Ar-H), 5.0 (OH), 3.85 (s, 2H, S-CH2), 3.73 (s, 3H, methoxy). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole, C=N),153.07-115.8 (Ar-C), 143.208(C=N), 56.2 (Methoxy), 40.9 (S-CH2). Mass : m/z = 356.1 (M-1) 2-(benzo[d]oxazol-2-ylthio)-N′-(3-nitrobenzylidene)acetohydrazide (IVi):
Mol. Formula : C16H12N4O4S Mol. Weight : 356 Solubility : Freely soluble in Methanol & Alcohol TLC solvent : n-Hexane: Ethyl acetate (3:7) IR, Cm-1 (KBr) : 3406.07 (N-H, str.), 3011.79 (Ar-CH, str.), 1622.47 (C=N), 1579.79 (N-H out of plane), 1184.38 (C-S). I H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 8.6 to 7.6 (m, 9H, Ar-H), 3.85 (s, 2H, S-CH2).
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C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N),163.04-124.18 (Ar-C), 143.208(C=N), 40.9 (S-CH2). Mass : m/z = 356 (M+1) 2-(benzo[d]oxazol-2-ylthio)-N′-(3-ethoxy-4-hydroxybenzylidene)acetohydrazide (IVj): 13
Mol. Formula : C18H17N3O4S Mol. Weight : 371.09 Solubility : Freely soluble in methanol & alcohol TLC solvent : n-Hexane: Ethyl acetate (3:7) IR, Cm-1 (KBr) : 3434.85 (N-H, str.), 3037.34 (Ar-CH, str.), 1731.76 (C=O, str. amide), 1618.95(C=N), 1165.76 (C-S). I H-NMR, δ ppm : 8.1 (s, 1H, N=CH), 8.0(s, 1H, NH-N), 7.5 to 7.1 (m, 7H, Ar-H), 5.0 (OH), 3.85 (s, 2H, S-CH2), 3.98, 1.33 (ethoxy protons). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N), 161.04-114.9 (Ar-C), 143.208(C=N), 65.0 (Ethoxy), 40.9 (S-CH2). Mass : m/z = 371 (M+1) 2-(benzo[d]oxazol-2-ylthio)-N′-((1H-indol-3-yl)methylene)acetohydrazide (IVk):
Mol. Formula : C18H16N4O2S Mol. Weight : 352.4 Solubility : Freely soluble in Methanol & Alcohol TLC solvent : n-Hexane: Ethyl acetate (3:7) IR, Cm-1 (KBr) : 3434.85 (N-H, str.), 3037.34 (Ar-CH, str.), 1731.76 (C=O, str. amide), 1618.95(C=N), 1165.76 (C-S). I H-NMR, δ ppm : 7.5 (s, 1H, N=CH), 7.0(s, 1H, NH-N), 7.5 to 7.1 (m, 9H, Ar-H), 3.85 (s, 2H, S-CH2). 13 C-NMR, δ ppm : 173.24(C=O), 165.31 (Benzoxazole C=N),158.04-115.18 (Ar-C), 144.8(C=N), 40.9 (S-CH2). Mass : m/z = 353.12 (M+1) Anti Bacterial Activity All the synthesized derivatives were characterized and screened for their anti bacterial activities by agar diffusion method. The standard cultures of grampositive bacteria Bacillus subtilis and gramnegative bacteria Enterobacter aerogenous, Proteus mirabitus, Klebsiella pneumonia, Salmonella paratyphi and Escherichia coli species. All the glassware were cleaned by a suitable cleansing agent and sterilized at 1210C and 15 lb/inch2 for 15 minutes. A volume of 25 ml of sterile hot agar medium was poured in each plate and allowed to harden on a level surface. The compounds were
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screened for antibacterial activity against Salmonella paratyphi, Enterobacter aeruginus, Proteus mirabitus, Klebsiela pneumonia, Escherichia coli and Bacillus subtilis in nutrient agar medium. The agar plates were inoculated with 24 hrs test cultures by spreading uniformly with sterile cotton swabs. The plates were then allowed to dry in the inverted position in an incubator for 30 min. Afterwards they were removed and bores were made on the medium using sterile borer. A volume of 0.1 ml of test solution was added to respective bores. Streptomycin at a concentration of 50µg/ml was taken as standard reference. Only
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DMSO was taken in a bore as control. The petriplates were kept in the refrigerator at 40C for 15 min for diffusion to take place. Afterwards they were incubated at 370C for 24 hrs and zones of inhibition were observed and measured using a
Compound code
ISSN 2349-7750
scale. Each experiment was carried out in triplicate and the mean diameter of inhibition zone was recorded. The various anti bacterial results of synthesized compounds are shown in the Table no 2.
Table 2: Anti bacterial activity by zone of inhibition Zone of Inhibition(mm) Gram –ve S. paratyphi
P. mirabitus
E. aerogenus
K. pneumoniae
Gram +ve E.coli
B. subtilis
IV a
8
7
8
6
8
5
IV b
13
10
12
11*
8
9
IV c 6 7 5 5 6 IV d NA 5 NA 8 8 IV e 14* 9 17* 10 11 IV f 12 5 11 4 6 IV g NA 4 7 NA 9 IV h 5 9 8 9 8 IV i 11 12* 14 7 5 IV j 9 NA 7 6 7 IV k 6 6 8 5 12* Streptomycin 18 15 20 14 15 (50µg/ml) NA = No activity. * = Significant activity, Concentration of test compound = 50µg/ml.
NA 7 10* 8 6 4 8 5 7 13
Table 3: Percentage Inhibition values for antibacterial activity % Inhibition Gram –ve
Compound code S.paratyphi
P.mirabilis
Gram +ve
E.aerogenous
K.pneumoniae
E.coli B.subtilus
IV a IV b
44.4 72.2
46.6 66.6
40 60
42.8 78.5*
53.3 53.3
38.4 69.2
IV c
33.3
46.6
25
35.7
40
NA
IV d IV e
NA 77.7*
33.3 60
NA 85*
57.1 71.4
53.3 73.3
53.8 76.9*
IV f
66.6
33.3
55
50
33.3
61.5
IV g
NA
26.6
35
NA
50
46.1
IV h
27.7
60
40
64.2
53.3
50.7
IV i
38.8
80*
70
28.5
27.7
38.4
IV j
50
NA
35
42.8
46.6
61.5
IV k
33.3
40
40
35.7
80*
53.8
Streptomycin (50 µg/ml)
100
100
100
100
100
100
NA = No activity, * = Significant activity. Concentration of test compound = 50 µg/ml.
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Fig 1: Graphical representation of anti bacterial activity by zone of inhibition
Fig 2: Graphical representation of anti bacterial activity by % inhibition
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Klebsiella pneumonia and the compound (IVk) was effective against Escherichia coli. The nitro group containing compound (IVi) was effective against P. mirabilis. The compound (IVb) was more effective against Colleotrichum coffeanum. Compounds (IVe) and (IVj) were effective against Aspergillus niger. ACKNOWLEDGEMENT: The authors wish to thank the management of Nalla Narasimha Reddy Education Society’s Group of institutions, School of pharmacy, Hyderabad, Telangana, India for providing necessary equipment for research, facilities and support.
Fig 3: Anti bacterial activity for standard and control
Fig 4: Anti bacterial activity of compounds IVd, IVe, IVf CONCLUSION: Introduction of benzoxazole moiety, in the synthesis and biological evaluation of novel 2mercaptobenzoxazole derivatives, the synthesized products was characterized by IR, NMR, mass and evaluated for anti bacterial activity by agar diffusion method. The compounds were screened for their anti bacterial activity. Among all the compounds synthesized, the furan containing compound (IVe) and chlorine containing compound (IVb) were effective against S. paratyphi. The compound (IVe) was also effective against E. aerogenous and B. subtilis. The compound (IVb) was effective against
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REFERENCES: 1.Srikanth. L; Naik. U; Jadhav. R; Raghunandan. N; Rao. J.V; Manohar. K. “Synthesis and evaluation of new phenylamino-thiadiazolo-oxadiazolo-1,3benzoxazoles for their antifungal and antiinflammatory activity”, Der Pharma Chemica, 2: 2010; 231-233. 2.Horton. D; Bourne. A; Smyth. G.”The Combinatorial Synthesis of Bicyclic Privileged Structures or Privileged Substructures”, Chemical Reviews,103: 2003; 893-896. 3.Alan R.K; Ming.Q; Daming. F; Guifeng. Z; Michael. C; Karen.W. “Synthesis of 1,2,4-TriazoleFunctionalized Solid Support and Its Use in the Solid-Phase Synthesis of Trisubstituted 1,2,4Triazoles”, Journal of Chemical Reviews, 96: 1996; 555-559. 4.Turker. L; Sener. E; Yalcin. I; Akbulut. U; Kayalidere. I. “QSAR of some antigungal active benzoxazole using the quantum chemical parameters”, Scientia Pharmaceutica: 58: 1990; 107111. 5.Devinde. R; Jacob. B; Sean. M. “Synthesis and Evaluation of Anticancer Benzoxazoles Related to UK-1”, Bioorganic & Medicinal Chemistry,10: 2002; 3997-4001 6.Srinivas. A; Jukanti. R; Vidyasagar. J; Ganta. R; Manda.S. “Synthesis and in vivo anti-inflammatory activity of a novel series of benzoxazole derivatives”, Der Chemica Sinica, 1: 2010; 157-160. 7.Kohli. P; Srivastava S.D; Srivastava S.K. ”Synthesis and biological activity of mercaptobenzoxazole based thiazolidinones and their arylidenes”, Journal of the Chinese Chemical Society, 54: 2007; 1003-1010. 8.Megumi.Y; Yasuo. S; Kazuko. K; Pukio. K, Tomoko. S and Takashi. W. Chemical and Pharmaceutical Bulletin, 46: 1998; 445-448. 9.Mathews. J; Craig. W; Mark. R.J; Leeie.T; Thorp. D, Thorantan. P; Lockhert. Journal of the Chemical. Society, Dalton Transactions, 8: 1996; 1531-1534.
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